Document how to prevent YAML-injection in Chart Templates? [2]

[meeque]

This is an alternative PR for #2154.

I had the impression that previous PR #2158 didn't fit well with existing contents of the "YAML Techniques" appendix.

This PR attempts to address this by editing that appendix more heavily. Major changes are:

• Split YAML types section into three:
  • Strings
  • Other Scalars
  • Collections
• Move Collections section to the end of these, since collections often use scalars.
  (Not sure about this decision, can revert if original order is deemed preferable.)
• For each section about YAML types, add a subsection about generating these types from dynamic values.
• Add a security considerations section at the the bottom.

I know this is a big edit, sorry for the spam. But I feel it conveys what template authors need to know about generating YAML.

[meeque]

Note: I've tried to stick to the existing writing style of the appendix. I know that it contradicts the style-guide, but imho that should be addressed through a separate issue...